1. Field of the Invention
This invention relates generally to the storage of speech in memory. More particularly, it relates to the managed compression of speech messages already stored in memory to provide a balance between the quality of the stored speech and sufficient memory availability for new messages as required.
2. Background of Related Art
Voice messaging has become an everyday requirement in today's society. Early voice messaging apparatus comprised magnetic cassette tapes which recorded a significant amount of voice messages, e.g., 60 minutes of voice messages. However, cassette tapes were disadvantageous because of the mechanics and time required to fastforward and rewind the cassette tapes to the storage points of individual messages. Moreover, cassette tape voice message systems were not best suited in a business environment where multiple persons may utilize a common telephone switching system or voice messaging system.
More recently, particularly as the size of memory has increased in density while at the same time decreased in price, digital voice messaging systems have become commonplace. Voice compression techniques such as linear predictive coding (LPC) or code-excited linear predictive (CELP) coding utilized by some conventional digital voice messaging systems maximized the length of voice messages which could be stored in the finite amount of memory provided in such apparatus. Compression techniques have been developed for use with digital voice messaging systems which in general reduce the number of bits necessary to represent voice message data. In general, the greater the compression, the greater the degradation in the quality of the speech stored in the memory.
Most recently, variable ratio voice compression techniques have been employed in voice messaging systems to maximize voice quality. Using variable ratio voice compression, the first message or so received by a voice messaging apparatus is typically stored in a memory at a lowest compression ratio, if compressed at all, and thus at the highest voice quality possible. "Compression ratio" as used herein refers to the ratio of the number of bits representing the voice message in linear form divided by the number of bits used to represent the same voice message in compressed or coded form. As new voice messages are stored, higher compression techniques are employed to compress new voice messages using higher compression ratios as the memory fills. This leads to a generally inverse relationship between the amount of memory utilized and the voice quality of the stored voice messages. The newest voice messages stored by these devices are stored using the highest compression ratios with respect to the oldest stored voice message. Moreover, the oldest stored voice messages utilize the most memory on a length-of-message basis with respect to the newest voice messages and are typically of the highest quality whether or not subsequent voice messages are stored. Thus, the stored voice quality of the newer voice messages suffers at the expense of the least compressed, not yet deleted, higher voice quality, older voice messages requiring more relative memory for storage
FIG. 9 shows a prior art voice messaging apparatus. A signal from a microphone or other analog signal source 806 is input to a codec 804 for conversion to .mu.-law or A-law pulse code modulated (PCM) data. The .mu.-law or A-law PCM data is output to a variable compression ratio encoder 803a. The codec 804 also receives PCM data from a variable compression ratio decoder 803b and converts the same into an analog signal output to speaker 808 for playback. A processor 802 controls the encoder 803a and decoder 803b including the compression ratio of the variable compression ratio encoder 803a and decoder 803b. Processor 802 also controls storage of the compressed (encoded) speech data from the variable compression ratio encoder 803a into memory 800, and controls the retrieval of compressed speech data from memory 800 and output of the same to the variable compression ratio decoder 803b.
FIG. 10 depicts the generally linear relationship between the compression ratio of the variable compression ratio encoder 803a and the amount of physical memory utilized in memory 800. As stored voice messages build up in memory 800 as represented by percentage on the vertical axis in FIG. 10, the compression ratio of the variable compression ratio encoder 803a is increased, causing the quality of the stored speech (voice) messages to decrease. FIG. 11 similarly depicts the variation in stored speech quality as the memory usage increases.
While the use of variable compression ratio speech encoders and decoders increases the amount and length of voice messages which can be stored in memory, the capacity of the memory is maximized by the conventional voice messaging system at the expense of the quality of the stored speech. Thus, as the volume of stored messages increases, the quality of the most recently stored speech messages decreases.